It is known in the automotive arts to couple a catalytic hydrocarbon reformer to the exhaust system of an internal combustion engine, and especially a diesel or other compression-ignited engine, in order to regenerate one or more pollution control devices in the exhaust stream. Such engines are provided typically with a diesel particulate filter (DPF) for collecting soot particles; and also with an LNT for adsorbing oxides of nitrogen generated during certain engine operating modes. Known LNTs are highly efficient at removing nitrogen oxides from an exhaust stream but typically have rather low capacity, requiring regeneration at short intervals of engine operation, for example, once per minute. Reformate, containing high levels of reducing agents, principally molecular hydrogen (H2) and CO, is known to be effective in reducing nitrogen oxides to molecular nitrogen, which can then be swept out of the LNT by the passage of further exhaust gas. Typically, reformate is produced by a warmed-up reformer on a periodic schedule driven by the periodic need for LNT regeneration, which schedule may be intermittent, continuous, or only during certain modes of engine operation.
An operational problem arises during start-up mode of the engine and reformer. A diesel hydrocarbon reformer has a warm-up period wherein reforming is performed continuously. During this time, effluent from the reformer is only partially reformed and contains significant amounts of non-reformed hydrocarbons (HC) as well as H2 and CO. Further, not all of the H2 and CO being produced is consumed by the LNT, resulting in unacceptable levels of HC, H2, and CO being emitted in the engine exhaust stream during start-up. A known approach to solving this problem is to combust the residual HC, H2, and CO in the exhaust stream ahead of the LNT during start-up.
Another operational problem arises during normal pulsed operation of the reformer wherein a pulse is too long and thus produces more H2 and CO than is required for LNT regeneration, resulting in HC and/or CO “breakthrough” of the LNT. This problem cannot be resolved by combusting excess reformate ahead of the LNT unless there is an effective means to sense when breakthrough occurs. It is known to determine when regeneration is complete by mounting a lambda sensor downstream of the LNT to sense breakthrough of HC and/or CO. This approach is unsatisfactory because a) it requires some level of breakthrough in order to function; b) it negates any further regeneration of the LNT even if only one portion of the trap has suffered breakthrough; and c) even after breakthrough is sensed and reforming is terminated, all the residual HC and CO within the exhaust system downstream of a combustor must be expelled in the exhaust stream to atmosphere.
What is needed in the art is a means for preventing breakthrough HC and CO from reaching atmosphere.
It is a principal object of the present invention to improve the quality of engine exhaust emissions by reducing the tailpipe levels of HC, H2, and CO originating in an associated HC reformer while also reducing levels of the same species originating in the internal combustion engine.